It is often the case that aircraft are required to leave a planned track owing to conflicting air-traffic or adverse weather conditions. When this happens, it is necessary for the aircraft to return to the planned track (or to follow a new track) from an arbitrary location that may be distant therefrom.
There are three phases to returning to follow the planned track: intercept, capture and tracking. The intercept phase involves steering the aircraft onto a path which meets the target track. In the capture phase the aircraft is steered towards the target track such that its actual path converges on that of the planned route. The tracking phase maintains the aircraft's position on the track.
Conventional approaches for guiding an aircraft to a planned track are typically complicated and operate separate modes for each of intercept, capture and tracking. These modes will be carried out by different control systems. Such approaches increase the complexity of flight management systems and have the problem of ensuring a seamless and stable transition from one mode to another.
Alternative approaches having only a single control system generally involve increased complexity and are therefore computationally expensive.